JPEG Tone Curves / dynamic range
Our Dynamic Range measurement system involves shooting a calibrated Stouffer Step Wedge (13 stops total range) which is backlit using a daylight balanced lamp (98 CRI). A single shot of this produces a gray scale wedge from the camera's clipped white point down to black (example below). Each step of the scale is equivalent to 1/3 EV (a third of a stop), we select one step as 'middle gray' (defined as 50% luminance) and measure outwards to define the dynamic range. Hence there are 'two sides' to our results, the amount of shadow range (below middle gray) and the amount of highlight range (above middle gray).
To most people highlight range is the first thing they think about when talking about dynamic range, that is the amount of highlight detail above middle gray the camera can capture before it clips to white. Shadow range is more complicated; in our test the line on the graph stops as soon as the luminance value drops below our defined 'black point' (about 2% luminance) or the signal-to-noise ratio drops below a predefined value (where shadow detail would be swamped by noise), whichever comes first.
Note: this page features our new interactive dynamic range comparison widget. The wedges below the graph are created by our measurement system from the values read from the step wedge, the red lines indicate approximate shadow and highlight range (the dotted line indicating middle gray).
The a7 has a tone curve that's nearly identical to the other cameras in this group. Its measured dynamic range is identical to the Sony a99 full-frame SLT, which captures more highlights than the D600 and 6D, but drops to black fractionally earlier in the shadows.
DRO (Dynamic Range Optimization) is an adaptive algorithm that brightens the dark regions of images to give a more balanced result while retaining local contrast. Our test chart doesn't tell the whole story, as in the real world the camera breaks an image down into smaller areas, adjusting the tone curve for each. With that out of the way, you can see above that DRO pulls up the shadows, without sacrificing highlights.
DRO Real World Example
|Off||Auto||Lvl 1||Lvl 2||Lvl 3||Lvl 4||Lvl 5|
While not the most exciting example, above you can see what each DRO setting does in a high contrast situation. The 'Auto' setting pretty much nails it, using a value similar to Level 5 in this case.
In our own experiences with the a7, we almost never changed the DRO setting from Auto, as it usually does the job. If you run into a situation with more contrast than DRO allows you to cope with, then consider using the well-designed HDR feature.
The a7 allows you to lower the base ISO to 50, for those who want to utilize wider apertures or slower shutters speeds in bright light. As is usually the case, the trade-off for this is an abrupt transition in the highlights to clipped white about a stop sooner than ISO 100.